Can only text people who have texted me

Hi everyone! I have a Samsung Galaxy s3, and I just got Koodo service 2 days ago on Vancouver Island. It seems like the most things are working just fine, but I am having a major texting problem. With a few odd exceptions, I currently am only able to text people who have texted me before--as in, since I got Koodo. This is true for Canadian numbers and international numbers. I was on the phone with tech support for over 3 hours yesterday, and they could not fix the problem. Texts that I send to people who have not texted me don't even leave my phone. They just give me a failed message.

I was with Mobilicity before in Vancouver, and I had zero problems with texting. I would appreciate any insights whatsoever!

if I clear the cache, that won't affect my contacts at all, right. It will just delete the thread and messaging settings? as far as my phone being broken, all I'm saying is that it has to do with my signing up with Koodo, not that it's not broken.